Neurohistiocytosis of the Cerebellum: A Rare Cause of Ataxia.

Erdheim-Chester disease (ECD) is a non-Langerhans cell histiocytosis that affects multiple body organs, notably the skeletal system. We examined a 58-year-old man who presented with ataxia and T2 hyperintensity of the middle cerebellar peduncles and dentate nuclei without contrast enhancement on MRI brain. Workup for malignancy revealed "hairy kidneys" on CT scan of the abdomen, and excisional biopsy of the retroperitoneal mass for concerns of lymphoma revealed foamy histiocytes that tested positive for CD68 and negative for CD1a, confirming the diagnosis of ECD. Further genetic testing on excised tissue revealed BRAF (V600E) gene mutation that is present in 50% of ECD patients. Treatment was initiated with targeted therapy using the BRAF inhibitor Dabrafenib. X-ray of the lower extremities did not reveal sclerosis of the long bones, and bone scan with technetium 99 was negative except for a nonspecific tracer uptake in left calvarial bone with no corresponding CT changes or T1/T2 signal changes on MRI. His MRI brain revealed classic cerebellar involvement in ECD without other central nervous system (CNS) involvement. It has been postulated that bone involvement is almost universal in ECD; however, our patient with ECD had ataxia and cerebellar involvement without significant bone involvement, as evidenced by bone scan. This is a rare presentation of ECD affecting the CNS and sparing the skeletal system. It confirms the wide spectrum of presentation this multisystem disease can have.